Two epicenters triggered Japan's powerful Noto quake on New Year's Day

Key points learned

A magnitude 7.5 earthquake struck under Japan’s Noto Peninsula on Jan. 1, 2024, when a “double-initiation mechanism” applied enough energy from two different locations to breach a fault barrier—an area that locks two sides of a fault in place and absorbs the energy of fault motion, slowing it down or stopping it altogether. An international team of researchers led by UCLA graduate student Liuwei Xu and UC Santa Barbara professors Lingxin Meng and Chen Ji analyzed a previous earthquake swarm and identified a previously unknown barrier in the swarm area. The data collection methods the team used could help future research into the conditions and likelihood of double-initiated earthquakes.

The first seven months of 2024 have been so eventful, it’s easy to forget that the year began with a 7.5-magnitude earthquake that struck Japan’s Noto Peninsula on New Year’s Day. The quake killed more than 280 people and damaged more than 83,000 homes.

Geologists have now discovered that the quake started at almost the same time at two different points on the fault, allowing the seismic rupture to encircle and penetrate a resistant zone on the fault known as the barrier. This rare “double-initiation” mechanism applied extreme pressure from both sides of the barrier, resulting in a powerful release of energy and significant ground shaking across the Noto Peninsula.

The Noto earthquake was preceded by seismic waves, which are sequences of many small earthquakes that can sometimes lead to a larger catastrophic event. Using advanced seismic and geodetic techniques, the research team carefully analyzed the movements within the Earth during these waves that led to the earthquake.

The study, published in the journal Science, provides insight into the role of fault barriers, also known as roughness, in earthquakes, and will help improve seismic hazard assessments and earthquake prediction in the future.

Earthquakes occur when cracks in the Earth's crust, known as faults, allow blocks of rock on either side of the fault to move past each other. This movement is localized rather than continuous along the fault line, because the line is not flat or smooth, causing energy to dissipate and eventually stop moving.

A barrier is a rough area that locks the two sides of a fault in place. Barriers absorb the energy of the fault's motion, slowing it down or stopping it altogether. But a barrier can only absorb a limited amount of energy, and under the right conditions, the pent-up energy causes the barrier to violently break, creating strong shaking. A series of small earthquakes may not be enough to break a barrier, but if much stronger movement occurs on the fault, the rupture of the barrier will release all that stored energy.

Led by Lingxin Meng, an associate professor of Earth, planetary and space sciences at UCLA, Liuwei Xu, a graduate student at UCLA, and Chen Ji, a professor of geophysics at UC Santa Barbara, an international team of researchers from the United States, France, China and Japan analyzed geospatial data and seismic wave recordings to understand the relationships between the smaller quake swarm and the larger earthquake that followed. They identified a previously unknown barrier in the swarm area.

To their surprise, the New Year's earthquake began at almost the same time in two separate locations on the fault. Energy from each location traveled toward the barrier, causing violent ruptures and extremely strong shaking.

“The earthquake started in two places and then circulated around each other. The first one caused waves that spread quickly and triggered a different epicenter. Then the two parts radiated outward together and met in the middle where the barrier was and broke it,” Meng said.

The mechanism is similar to bending a pencil from both ends until it breaks in the middle.

Shaw et al. 2024

Illustration of the initial fault sequence of the 2024 Noto earthquake, showing the distribution of high-frequency seismic energy and subsequent aftershocks across the fault system.

The result was surprising because, although the double-start process, as it is known, has been seen in simulations, it has been extremely difficult to observe in nature. Double-start mechanisms require just the right conditions, which can be tuned in the laboratory but are less predictable in the real world.

“We were able to detect it because Japan has very good seismic monitoring stations and we also used GPS data and satellite radar data. We collected all the data we could find! And just by combining all of that data we got a really high resolution of this fault and were able to get to these fine details,” said Meng.

No data is collected on the vast majority of earthquakes, so it is possible that earthquakes with dual initiation mechanisms are more common than geologists realize.

“With better imaging and higher resolution, we may be able to identify more of these diseases in the future,” Meng said.

Earthquakes with double epicenters are more prone to strong shaking due to strong motion. The Ming Group plans to study future scenarios to learn about the conditions and likelihood of such earthquakes.

“Our results highlight the complex nature of earthquake initiation and the critical conditions that can lead to large-scale seismic events,” Meng said. “Understanding these processes is vital to improving our ability to predict and mitigate future earthquakes.”

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